Codeply is a responsive design playground & front end editor, which lets you run and preview HTML/CSS & Javascript code through a web browser. You might have seen similar tools which enables code editing in browser, some of them are listed below.

With Codeply you get integrated responsive design frameworks like Bootstrap, Foundation, SemanticUI etc. So you can run all those frameworks right in the browser and try it out with your HTML. Makes front end coding with frameworks a ton more easy. Other features include viewport resizing for responsive design testing, quick code editing support for frameworks and many more.